Image forming apparatus with sub-pixel position control

Abstract

In an apparatus for forming an electrostatic latent image in a form of dot for each pixel on a photoreceptor, each pixel has plural color component signals and a dot is formed for each of color component signals in the same imaging unit area. A signal processing circuit processes specific color component signals of pixels neighboring a target pixel to obtain a density distribution around the target pixel, and determines a recording position of the dot in the recording unit area for each of the plural color component signals on the basis of the density distribution.

What is claimed is: 
     
       1. An apparatus for forming a color image of pixels each having plural color component signals, wherein an electrostatic latent image is formed as an image dot for each color component signal in an imaging unit area for each pixel on a photoreceptor, the imaging unit area having plural dot forming positions, and the image dot for each color component signal being selectively formed at one of the dot forming positions, the apparatus comprising: beam generating means for generating a light beam to expose the photoreceptor to the image dot for each color component signal;   means for processing image data, said processing means including: means for obtaining a specific color component signal representing a density level of a specific color for each pixel on the photoreceptor from the plural component signals including at least one color component signal for a target pixel, means for processing specific color component signals of pixels neighboring the target pixel to obtain a first density distribution of neighboring pixels around the target pixel, means for determining a dot forming position of each color component signal of the target pixel in the imaging unit area of the target pixel based on the first density distribution of the neighboring pixels, and means for processing said at least one color component signal of the target pixel in response to the determined dot forming position to generate a modulating image signal for said at least one color component signal of the target pixel; and   means for modulating the light beam based on the modulating image signal so that the image dot of said at least one color component signal of the target pixel is formed at the determined dot forming position in the imaging unit area of the target pixel.   
     
     
       2. The apparatus of claim 1, wherein said plural color component signals are red R, green G, and wherein said specific color component signal includes a green color component signal. 
     
     
       3. The apparatus of claim 1, wherein said plural color component signals are red R, green G, and blue B, and wherein said specific color component signal N =R+2G+B. 
     
     
       4. The apparatus of claim 1, further comprising wave generation means for generating plural types of reference wave signals differing in phase to provide plural different modulating image signals so that an image dot can be formed at one of different positions in said imaging unit area in accordance with a reference wave signal phase, wherein said processing means includes means for selecting one of said plural types of reference wave signals based on said first density distribution. 
     
     
       5. The apparatus of claim 4, wherein said processing means includes means for dividing said target pixel into plural sub-pixels in a form of matrix (n rows×m columns) and for determining a second density distribution of said plural sub-pixels based on said first density distribution. 
     
     
       6. The apparatus of claim 5, wherein said processing means includes means for obtaining a center of gravity for each of the n rows means for selecting one of said plural types of reference wave signals for each of the n rows based on a position of said center of gravity. 
     
     
       7. The apparatus of claim 1, further comprising means for discriminating an image of said target pixel. 
     
     
       8. The apparatus of claim 7, wherein, when said discriminating means discriminates the image of said target pixel as a halftone image, said processing means conducts said determining of the dot forming position for only an achromatic component signal. 
     
     
       9. The apparatus of claim 7, wherein, when said discriminating means discriminates the image of said target pixel as a character image, said processing means conducts said determining of the dot forming position for all color component signals. 
     
     
       10. The apparatus of claim 4, further comprising means for discriminating an image of said target pixel, wherein said processing means includes means for changing a cycle period of said reference wave signals in accordance with a discrimination result of said discriminating means. 
     
     
       11. The apparatus of claim 10, wherein, when said discriminating means discriminates the image of said target pixel as a halftone image, the processing means uses a reference wave signal having a cycle period corresponding to two pixels. 
     
     
       12. The apparatus of claim 10, wherein, when said discriminating means discriminates the image of said target pixel as a character image, the processing means uses a reference wave signal having a cycle period corresponding to a pixel. 
     
     
       13. The apparatus of claim 1, wherein said processing means includes means for comparing an image signal of said target pixel with a first threshold value, and when said image signal of said target pixel is larger than said first threshold value, said processing means conducts the determining of the dot forming position. 
     
     
       14. The apparatus of claim 13, wherein said processing means includes means for comparing an image signal of said target pixel with a second threshold value, and when said image signal of said target pixel is smaller than said second threshold value, said processing means conducts the determining of the dot forming position. 
     
     
       15. The apparatus of claim 1, wherein said modulating means has a main scanning direction to scan the light beam on said photoreceptor and a sub scanning direction normal to said main scanning direction, and wherein said processing means includes means for conducting the modulation in the main scanning direction by selection of a reference wave signal and means for conducting the modulation in the sub scanning direction in accordance with an average density in said target pixel in the main scanning direction.